Modal Analysis

[Simon Radler]

Hi,

the 'Simbody Theory Manual' mentions that modal analyses could be performed using the dynamic, internal coordinate Jacobian. Is this actually possible with the current Simbody version?

Thank you!
Simon

[Michael Sherman]

You can obtain the system Jacobian using

SimbodyMatterSubsystem::calcSystemJacobian(). I believe the manual is referring to the eigenvalues of this matrix as the system modes.

Sherm

[Simon Radler]

Hi Sherm,

thanks for your reply. The documentation states that calcSystemJacobian() returns a mapping from generalized speeds to the spatial velocities of all the bodies. In contrast, the modal analysis requires the derivatives of the system equations of motion with respect to the generalized coordinates and speeds.
So unfortunately these Jacobians are different to my understanding.

Thanks anyway,
Simon

[Michael Sherman]

:facepalm: yes of course you're right. You want partial(acceleration)/partial(state). Simbody can provide acceleration(state) (fast) but you would have to differentiate that numerically to get the matrix. That's easy to do by hand or using Simbody's Differentiator class.

Sherm